Cytogenetic and cell genetic techniques will be employed in obtaining an understanding of fundamental biological principles potentially relevant to mental retardation. Gene mapping, principally of mouse and man, will be carried out to determine general principles regarding gene organization as well as to elucidate in detail the map of specific regions (such as human chromosome llp), especially relevant to mental retardation syndromes. Somatic cell genetic studies will be conducted of selected problems, such as cell fusion and cellular uptake of exogenous DNA, of potential importance to the analysis and management of inherited disorders. An assessment will be made of the influence of various biologic factors on the frequency of sister chromatid formation in vivo, in an attempt to understand their importance in disease and mutagenesis.